Allow Stripe Payout Account Type by Affiliate

Instead of choosing the Stripe account type (standard or express) for all affiliates, allow specifying that on an individual affiliate basis.

This is important (at least in the US) because US -> US payouts via Stripe are free but Stripe policy does not allow cross-border payouts with standard accounts, so you are required to use $2/month/affiliate express accounts for affiliates in other countries.

This is related to supporting Cross-border payouts (https://roadmap.couponaffiliates.com/boards/feature-requests/posts/support-for-cross-border-payouts-in-stripe), but I determined that Stripe’s undocumented policies do not allow such payments with Standard accounts. Hence the only solution for cross-border payments is using express accounts for those affiliates outside the host’s country.